Variable-pitch propulsor systems having a plurality of propulsor blades rotatably mounted to a rotary hub driven by the aircraft engine are known to be operatively connected to a mechanical, hydro-mechanical or even electrical blade pitch change actuation system disposed in an interior chamber within the hub. One type of pitch change system, commonly referred to as a linear actuation system, is disclosed, for example, in commonly-assigned U.S. Pat. Nos. 4,753,572; 4,936,746; 5,199,850; and 5,161,948. In such linear systems, rotation of a ball screw actuator causes a nut threaded thereabout to translate either forwardly or rearwardly along the ball screw, depending upon the direction of rotation of the ball screw. The ball screw nut is integral to a surrounding yoke, which is operatively connected to each of the blades through a plurality of articulating links, each of which connects to a rotatable trunnion in which a blade is mounted, such that translation of the yoke is converted to a rotation of each blade about its longitudinal axis. The pitch change actuator is typically driven by selectively activating a hydraulic or electric drive mechanism which serves to rotate the pitch change actuator relative to the rotating hub so as to create a differential motion and effect a rotation of each of the blades within their individual hub sockets about their longitudinal axes, thereby changing the pitch setting of the blade.
Blade pitch mechanisms actuated by electrical machines are shown in U.S. Pat. Nos. 5,183,387; 5,205,712; and 5,211,539, for example. In U.S. Pat. No. 5,183,387, a pair of electrical machines cause a relative displacement between corresponding movable input means to a mechanical actuator for actuating the blades. The first movable input means has the rotor of the first electrical machine attached thereto, while the second machine has its rotor attached to the second movable input means. Their stators are held stationary with respect to the rotors. Controllers provide excitation currents to selected windings of the stators to cause the relative displacement.
In U.S. Pat. No. 5,205,712, three electrical machines are used. A compound, polyphase AC reversible induction motor is controlled by excitation of one or the other of two separately excited polyphase alternating current generators. The reversible motor has a double winding that can be excited by one of the other.
A not-publicly known pitch change mechanism of assignee uses two inductive brakes, one for increased pitch and one for decreased pitch. This concept has its limitations, which are described in the above-referenced copending patent application filed on even date herewith, which problems are overcome by using two inductive machines, one brake and one motor connected to a single rotor assembly. The rotor assembly is connected to the ball screw gear train via a single feed-through gear. The brake is implemented as a unidirectional, DC induction machine that is very good at high speeds because of its high torque for slowing down the rotor. For low speeds, the bidirectional motor is implemented as an AC induction motor for low-speed, moderately high torque or low torque purposes.
In the particular implementation disclosed in the above-cited copending application filed on even date herewith, the ball screw-driven actuation system is designed to accurately control and maintain discrete fan blade pitch positions for a high bypass turbo fan engine. This system actually incorporates two electromagnetic devices, an AC induction motor and a DC induction brake for controlling the blade pitch variation. A solenoid-driven pitch lock mechanism may be used as the lock mechanism at the discrete positions.
The induction motor and brake rotors are mechanically locked to each other via a common rotor assembly and are mounted around and electro-mechanically linked to the engine fan output shaft. Under steady-state conditions, this linkage is normally locked, thus allowing the motor/brake rotor assembly to rotate at the same speed as the fan shaft. When a change in blade pitch is desired, the appropriate electromagnetic machine is selected and energized, torque equilibrium is established at the rotor assembly, and the pitch lock mechanism (a solenoid-driven disc brake at the rotor) is disengaged from the fan shaft by applying solenoid power. The resulting differential speed between the rotor assembly and fan output shaft is converted to blade pitch rotation via a feed-through gear, a ball screw/ball nut gear train and blade trunnion linkages. When pitch position is achieved, the pitch lock mechanism is re-engaged, power to the two induction machines is removed, and the rotor assembly once again resumes rotation at fan shaft speed until the next command for pitch change is issued by an engine controller.
This particular implementation places a substantial challenge to classically-derived control algorithms, as it requires that: 1) the motor/brake rotor assembly be disengaged and re-engaged within hardware capabilities, while simultaneously meeting position accuracy requirements; and 2) a scheme exist for selecting and energizing the appropriate induction machine for the proper duration and dynamic condition.